The invention relates to the forming of connections to the bonding pads on integrated circuit (IC) devices. Monolithic silicon IC devices commonly employ an array of aluminum film bonding pads usually disposed around the periphery of the individual chips. It is common practice to treat a silicon wafer containing a plurality of IC chips to simultaneously form bumps on all of the bonding pads. Typically, the bumps are composed of gold, silver or copper in the tape automated bonding(TAB) process. Copper tape fingers are bonded to the bumps after the chips are separated from the wafer. Alternatively, the bumps are composed of solder and the flip-chip assembly approach employed. Her the chip is inverted so that the solder bumps face the traces on a printed wiring PW substrate. The chips are pressed against the substrate and heat is applied to fuse the solder which melts and bonds the chip to the substrate.
In the TAB approach the tape fingers can be thermocompression (TC) bonded or they can be reflow soldered to the bumps. With reflow soldering, it is common to tin plate the copper tape fingers and to either employ gold bumps or gold plate the copper bumps. The fingers are pressed against the bumps and heat is applied to exceed the gold-tin eutectic. The gold and tin combine to form a eutectic solder that will bond the copper to the bumps.
Each of these prior processes suffer drawbacks. When solder bumps are employed their fusion causes them to lose their original geometry. Furthermore, solder bumps are not amenable to TAB because a PW substrate is required. The TC process for attaching metal fingers to the bumps involves substantial pressure. In many cases, particularly for high lead count IC chips, the total pressure can produce stresses that adversely affect reliability. In the case of tin plated TAB tape, special handling must be employed if the tape is to be stored. Furthermore the formation of tin whiskers can occur to the detriment of assembled devices. If a tin surface is exposed, inside a finished device, tin whiskers can form and short out the leads. Finally, the tin can electromigrate in the presence of applied voltages and produce lead shorting.